Hollow bodies that are pressure-tight for a predetermined medium, which is contained in the hollow interior of the hollow body during its intended use, can be provided for a variety of applications. One possible application is the use of the hollow body as a heat pipe. In this application, the medium is moved back and forth between a hot end and a cold end of the heat pipe due to a movement of the heat pipe; it absorbs heat at the hot end and gives off this heat at the cold end. This permits a dissipation of heat from the hot end.
This application is known, for example, from DE 44 05 091 A1. In that case, a hollow body embodied in the form of a heat pipe is used as a piston rod of a Scotch yoke motor. In it, the piston rod connects the piston to the Scotch yoke. The liquid contained in the piston rod, which can be water for example, absorbs heat at the piston end of the piston rod, evaporates as a result, and is then moved in the direction of the Scotch yoke by the movement of the Scotch yoke and thus of the piston rod. This end of the piston rod is cooler or can actually be cooled so that the liquid condenses there and thus gives off heat. The piston rod is integrally connected to the Scotch yoke. The piston is screwed to a plug that is inserted into the open end of the piston rod, after the liquid has been introduced into the piston rod. Such a screw connection, however, can become leaky due to the high stresses that occur in a Scotch yoke motor. This is further exacerbated by the fact that the rapid back and forth movements cause vibrations to occur, as a result of which the screw connection can gradually come loose and needs to be retightened at particular intervals. Also the plug seal itself can become leaky due to the vibrations and the associated wear.
Another application for a hollow body that is pressure-tight for a predetermined medium is the use as a valve for a reciprocating piston internal combustion engine. Such a valve is known from DE 2 324 850. In the method described therein, the valve shaft is welded to the valve disc part in a gas-tight fashion. Then a heat transfer liquid, in particular sodium, is placed into an inner cavity of the valve shaft in a liquid state. The heat transfer liquid must then be cooled so that it assumes the solid aggregation state. After a laborious cleaning, the valve-closing part is inserted from the end into the inner cavity of the valve shaft and then fastened to the valve shaft by means of electron beam welding. The above-described method is very labor-intensive since it requires many method steps and during production, it requires multiple heating with subsequent selective cooling.
DE 19 22 748 has disclosed a method and device for filling a heat pipe. As a disadvantageous option when producing a heat pipe, it discloses freezing the working liquid, evacuating the heat pipe, i.e. providing it with a vacuum, and then welding it by means of electron beam welding. With regard to the freezing, it is considered to be disadvantageous that frost can form on the circumferential surface of the heat pipe, which interferes with the electron beam welding machine. This publication therefore assumes that the welding process is electron beam welding, which makes it necessary to act on the interior of the heat pipe with vacuum since electron beam welding takes place in a vacuum. The freezing of the working liquid therefore serves to hinder a volatilization, i.e. an evaporation of the working liquid under vacuum.
The above-mentioned publication also proposes initially placing the working liquid in an ampule and then inserting the ampule into the cavity before the heat pipe is welded shut. After the heat pipe is closed, the ampule must be destroyed in order to release the working liquid. It also proposes embodying the ampule out of glass or plastic.
In this method, it is disadvantageous that the ampule must be embodied as relatively stable so that it does not break during production of the vacuum for the electron beam welding. With a relatively stable and solidly embodied ampule, it is not always possible to ensure that it will be reliably possible to destroy it after the closing of the heat pipe. Particularly with an ampule made of glass, it is disadvantageous that after the ampule is destroyed, glass splinters remain in the cavity and may possibly damage an inner surface of the cavity. For a heat pipe that is used as a piston rod of a reciprocating piston engine, it is not acceptable to leave glass splinters inside the piston rod.
The object of the invention, therefore, is to provide an easy-to-perform method for producing a hollow body that is pressure-tight for a predetermined medium in which the medium is prevented from escaping from the cavity of the hollow body during production.
Another object of the invention is to more economically embody known methods for producing a hollow body that is pressure-tight for a predetermined medium and to make them easier to perform.
Another object of the invention is to provide a method for producing a hollow body that is pressure-tight for a predetermined medium in which it is also possible for highly volatile and in particular nonmetallic mediums to be used as the predetermined medium.
Another object of the invention is to provide a piston rod with a hollow body that is pressure-tight for a predetermined medium, which is easy to produce.
Another object of the invention is to provide a piston rod with a hollow body that is pressure-tight for a predetermined medium, in which a sufficient quantity of medium is present in the hollow body.